The invention relates to a traction converter having a line-side four-quadrant controller, which has power semiconductors which can be switched off as converter valves, each of which has a power diode connected electrically back-to-back in parallel with it, and having a regulation device which is linked on the output side to control connections for the power semiconductors which can be switched off.
In the case of traction drives, one aim is to keep the vehicle wattles-component demand as low as possible. A further aim is to ensure that signal circuits and telecommunications links are not influenced by current distortion, such as that which can occur in particular in the case of converter drives. In order to be as compatible as possible with the power supply system, traction converters are equipped with a line-side pulse-controlled converter. A line-side pulse-controlled converter such as this for a traction converter which generates a required DC voltage from a single-phase line voltage is referred to as a four-quadrant controller. The motors in the traction drive are fed via a load-side pulse-controlled inverter, which is fed from the DC voltage.
Power semiconductors which can be switched off are used with corresponding freewheeling diodes as converter valves for the four-quadrant controller. These freewheeling diodes are power diodes which are each connected electrically back-to-back in parallel with the corresponding power semiconductor. Power semiconductors and associated power diodes are normally arranged in a power semiconductor module. By way of example, Insulated Gate Bipolar Transistors (IGBT) are used as power semiconductors which can be switched off. pin diodes are used as power diodes, since the reverse voltages which occur are more than 100 V.
These pin diodes have a conducting-state voltage of about 2 V. The conducting-state voltage of high-blocking capacity pin diodes is higher than this, typically 4 V. In the case of pin diodes, the transition from the conducting-state range to the reverse-biased range does not take place instantaneously, since the charge that is stored in the pn junction must first of all be dissipated. The time required to do this is the storage time, which becomes greater the higher the conducting-state current was before the transition. For power diodes, this storage time is in the microsecond range.
During operation of the four-quadrant controller, in comparison to the pulse-control inverter on the motor side, the power diodes carry current for the majority of the time, and the power semiconductors carry current for the minority of the time. This leads to overloading of the power diodes when the power semiconductors are utilized well. In the past, this problem has been solved by using power semiconductor modules with an enlarged diode chip area.
Controllable diodes are known from the publication entitled: “Power Diodes with Active Control of Emitter Efficiency” by Dirk Drücke and Dieter Silber. These controllable diodes, which are also referred to as emitter controlled diodes (ECD), can assume one of two states by means of a control signal. In one stage, the amount of stored charge is very high and the conducting-state voltage is very low, while in the other state the amount of stored charge corresponds to a rapidly switching diode and the conducting-state voltage is higher. The first-mentioned state is reached as soon as a control signal that is applied to the control connection of the diode is low. When this control signal level changes from low to high, then the diode changes to the state with a low storage charge.
The invention is now based on the object of improving a known traction converter having a line-side four-quadrant controller in such a manner that there is no longer any need to use power semiconductor modules with an enlarged diode chip area.